1) Field of the Invention
The present invention relates to a method and device each of which controls the temperature of a thermal fixing machine suitably used in printers, facsimile apparatuses, copying machines, or the like using an electrophotographic technique to fix an toner image onto a recording medium.
2) Description of the Related Art
An electrophotographic recording apparatus has been developed, that records an image by transferring a toner image of resin powder on a recording medium (for example, a recording paper), and melting to permeate thermally it into the recording medium so that the toner image is fixed on the recording medium.
The electrophotographic recording apparatus includes a thermal fixing machine which melts thermally a toner image of resin powder on a recording medium such as a sheet of paper to permeate it into the recording medium so that the toner image is fixed thereon.
FIG. 13 is a cross sectional view showing diagramatically the structure of a thermal fixing machine. Referring to FIG. 13, the thermal fixing machine includes a heat roller 111, a heater 112, a thermistor (temperature sensor) 113, a spring 114, a backup roller 115, and a separation pawl 116.
The heat roller 111 is formed of an aluminum pipe with a resin film such as Teflon (a trade name) coated over the external surface thereof, and a heater 112 arranged in the inner space of the aluminum pipe. The heater 112 heats the heater roller 111. The thermistor 113 is a variable resistor which varies its resistance with temperature. The thermistor 113 is arranged near to the heat roller 111 to detect the temperature of the heat roller 111.
The backup roller 115 is subjected to a bias force by the spring 114 to provide a constant pressure against the heat roller 111. A sheet of paper (recording medium) 120 passes between the backup roller 115 and the heat roller 111.
The separation pawl 116 peels the sheet 120 out of the heat roller 111.
When a sheet 120 with a toner image transferred thereon is introduced between the heat roller 111 and the backup roller 115, a toner image is fixed on the sheet 120 by applying heat from the heat roller 111 and the pressure of the backup roller 115.
A suitable amount of heat is needed to perform a toner heating by the thermal fixing machine. In other words, a thermal fixing machine in short of heat amount or at low temperature occurs the so-called cold offset in which a toner image is transferred on the roller 111 of the thermal fixing machine. By contraries, a thermal fixing machine heated excessively or at an excessive temperature occurs a hot offset in which toners melted stick to the heat roller 111.
Such a poor fixing or offset causes degraded image quality and a device contaminated. The thermal fixing machine at an excessively high temperature increases heat dissipation, thus causing ambient environmental deterioration and wasted power dissipation. Hence, the temperature of a thermal fixing machine should be controlled to the minimum.
In order to resolve the above demands, the control device shown in FIG. 14 generally controls a temperature heated by the heater 112 of a thermal fixing machine.
The control device is constituted of a heater 112 and a thermistor 113 shown in FIG. 13 in addition to resistors 101 to 103, a variable resistor 104, an operational amplifier 105, and a heater drive circuit 107.
The thermistor 113, corresponding to the thermistor 113 shown in FIG. 13, is a variable resistor which varies its resistance with temperature. The thermistor 113 produces a voltage corresponding to the temperature of the heat roller 111 to the operational amplifier 105.
A combination of the resistors 102 and 103 and the variable resistor 104 inputs a reference voltage to the operational amplifier 105. The reference voltage corresponds to a temperature (control target temperature ) to control a heating temperature of the thermal fixing machine.
Hence, the operational amplifier 105 compares a temperature of a heat roller 111 detected by the thermistor 113 with a reference temperature to output a voltage corresponding to its difference.
The heater drive circuit 107 receives an output (comparison result) from the operational amplifier 105 and subjects the heater 112 to an on/off drive control by supplying electric power to the heater when the heater is at less than a predetermined temperature or halting power supply to the heater when the heater is at more than a predetermined temperature. The heater drive cicuit 107 controls the temperature of the heat roller 111.
In order to cope with different suitable temperatures due to a change in use environment and record medium, the latest devices includes an A/D converter 121 and a CPU 122, instead of the resistors 102 to 104 and the operational amplifier 105 shown in FIG. 14, so that a predetermined temperature can be varied according to conditions, as shown in FIG. 15.
In the control device of a thermal fixing machine, shown in FIG. 15, the A/D converter 121 subjects temperature detection information (analog value) from the thermistor 113 to a digital conversion and the CPU 122 compares the digital value with a reference value (threshold), so that the heater 112 is on/off controlled.
In the CPU 122, a first threshold is set to subject the heater 112 to an on (lighting on) control and a second threshold larger than the first threshold is set to subject to an off (turning off) control. Hysteresis forming a difference between the first threshold for light-on control and the second threshold for light-off control prevents a heater chattering (a unstable factor shortening the operational life of a heater 112).
In the above configuration, the control device (CPU 122) of a thermal fixing machine shown in FIG. 15 operates in accordance with the flow chart shown in FIG. 16.
The A/D converter 121 subjects an analog voltage as a temperature detection information inputted from the thermistor 113 to an A/D conversion process. The CPU 122 samples digital values from the A/D converter 121 (step S1). The CPU 122 compares a sampled digital value with the first threshold (step S2).
In the step S2, when the digital value sampled is less than the first threshold, the CPU 122 outputs a control signal to the heater drive circuit 107 to subject the heater 112 to an on control (step S3) to sample the following digital value (step S1).
When it is judged that a sampled digital value has been equal to or more than the first reference value in the step S2, it is compared with the second threshold larger than the first threshold (step S4).
In the step S4, when it is judged that a digital value sampled is larger than the second threshold, the CPU 122 outputs a control signal to the heater drive circuit 107 to off-control the heater 112 to the previous control state (step 5), so that the following digital value sampling is performed (step S1).
Moreover, in the step S4, when it is judged that a digital value sampled is equal to or smaller than the second threshold (a digital value sampled is between the first threshold and the second threshold), the CPU 122 outputs a control signal to the heater drive circuit 107 to maintain the condition of the heater 112 in the previous state (step S6), thus sampling the next digital value (step S1).
However, in the prior art control means for a thermal fixing machine, a temperature sensor such as the thermistor 113 has a non-linear characteristic to variations in temperature shown in FIG. 17. The temperature sensor is required to detect a wide detection region ranging from a room temperature to about 140.degree. C. of a fixing temperature. Consequently, when only the temperature sensor is used to detect the wide detection region, it is not always used over higher resolution regions of the A/D converter 121. Therefore, there is a problem that the low detection accuracy of a temperature sensor which controls a fixing temperature varies its temperature control value.
For example, as shown in FIG. 17, since a voltage variation as a detection value to a temperature variation is large over temperatures ranging from a room temperature to about 100.degree. C., the A/D converter 121 can accurately detect a digital value as a temperature detection value. However, a small voltage variation as a detection value to a temperature variation in the vicinity of a fixing temperature of about 140.degree. C. makes low the accuracy of the A/D converter 121, whereby the accuracy of a temperature detection value to control the fixing temperature becomes lower.
Since the least significant bit of the A/D converter 121 is very unstable, the hysteresis width between the thresholds of heater on/off operation points which is decided to prevent a heater chattering becomes small, thus resulting in a large temperature variation.
Moreover, in order to improve a detection accuracy to a required detection temperature region, using different circuits over different detection temperature ranges or an A/D converter with higher resolution boosts the device manufacturing cost.